According to Wikipedia:
A blue moon can refer to either the third full moon in a season with four full moons, or the second full moon in a month. Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules about when to insert such anintercalary or embolismic ("leap") month, and what name it is given; e.g. in the Hebrew calendar the month Adar is duplicated. The term "blue moon" comes from folklore. Different traditions and conventions place the extra "blue" full moon at different times in the year. In the Hindu calendar, this extra month is called 'Adhik(extra) masa (month)'. It is also known as purushottam maas, so as to give it a devotional name.
Blue Moon Calendar - 2 Full Moons in one month
from http://www.moongiant.com/Blue_Moon_Calendar.php
Year First Full Moon of month Second Full Moon of month (Blue Moon)
2009 Full Moon – December 2, 2009 Blue Moon – December 31, 2009
2012 Full Moon – August 2, 2012 Blue Moon – August 31, 2012
2015 Full Moon – July 2, 2015 Blue Moon – July 31, 2015
A blue moon will grace Neil Armstrong’s funeral service this Friday. It is a great coincidence that the once-in-two-year blue moon raising occurs on the same day, the great astronaut’s funeral service is planned. Once the funeral service will take place in Cincinnati, blue moon will pop into the Western Hemisphere sky on Friday. Armstrong, the first man to set foot on the moon died last Saturday at his age of 82. The world is still mourning the demise of the great astronaut, an American hero. More of this Article here.
